The invention relates to an aluminoborosilicate glass having a particularly low density and a high specific modulus of elasticity, and to advantageous uses of such a glass.
LCD displays are becoming increasingly widespread, not only as computer screens but also as large-format flat screens. TFT-LCD displays (active matrix thin-film transistor LCDs), especially, have low power consumption and are therefore used in many applications, for example in notebooks, in flatscreens, in digital cameras and the like. The display substrate usually consists of a glass plate in each case.
Such substrates must meet high standards. In addition to high thermal shock resistance and good resistance to the aggressive chemicals used in the production of flat screens, the glasses should also have a wide spectral range (VIS, UV), high transparency, and a low density in order to save weight. Furthermore, using them as a substrate material for integrated semiconductor circuitry, for example in TFT displays, requires that they be thermally adapted to the silicon thin-film material. When largely crystalline silicon layers are produced by high-temperature treatment at temperatures above 700° C., or by direct deposition in CVD processes, a substrate is required which has a low thermal expansion coefficient of less than 3.2·10−6/K, if possible. Another condition for applications in the field of display and photovoltaics technology is the absence of alkali ions. Sodium oxide concentrations of below 1000 ppm resulting from production-related factors are still tolerable with regard to the “poisoning” effect resulting from the diffusion of Na+ into the semiconductor layer.
However, for use in large-format displays, it is particularly essentially that the substrate has a high elasticity modulus, a low density and a high specific modulus of elasticity.
It is also desired that suitable glasses can be commercially produced in a cost-efficient manner and at a sufficient level of quality (no bubbles, knots or occlusions), for example in a float glass plant or in a down-draw process. The production in drawing processes of thin (<1 mm), streakless substrates of low surface waviness, especially, requires that the glass have a high devitrification stability. In order to counteract any disadvantageous compaction of the substrate on the semiconductor microstructure during production, particularly in the case of TFT displays, the glass also needs to have a suitable temperature-dependent viscosity curve. This means that, as far as its thermal process and form stability are concerned, it should have a viscosity in the melting and processing range which is not too high, yet also have a sufficiently high transformation temperature, i.e. Tg≧700° C.
Many glasses used especially for displays such as LCDs and TFT-LCDs are described in the prior art.
A number of alkali-free aluminoborosilicate glasses are known from WO 02/060831 which have a relatively low density of less than 2.4 g/cm3 in some cases, but a specific modulus of elasticity that is always below 30 GPa·g−1·cm3.
Other alkali-free aluminoborosilicate glasses are known from DE 100 00 836 A1, but these all have a density greater than 2.4 g/cm3, which is considered disadvantageous in portable applications, especially, such as notebooks.
A number of other aluminoborosilicate glasses for such applications and which have a high modulus of elasticity and a high specific modulus of elasticity are known from U.S. Pat. No. 6,537,937 B1.
However, the transformation temperature of these glasses is less than 700° C. in all cases, which is considered disadvantageous.
Other glasses of a similar kind are known from U.S. Pat. No. 5,851,939; U.S. Pat. No. 6,060,168; DE 100 64 804 A1; U.S. Pat. No. 6,319,867 B1; EP 0 672 629 A2; DE 196 80 967 T1; JP 2004-91244 A; and from JP 08-295530 A.
However, all of these fail to meet the requirements referred to at the beginning.